Objective Lens Element

ABSTRACT

An objective lens element having excellent compatibility with optical discs having different base material thicknesses is provided. The objective lens element has optically functional surfaces on an incident side and an exit side. At least either one of the optically functional surfaces on the incident side and the exit side includes a diffraction portion which satisfies at least either one of the following formulas (1) and (2): 
       θ 1 ×θ 2 &lt;0 (“×” represents multiplication)   (1),
 
       (sin θ 2 )/λ2 2 =−(sin θ 1 )/λ 1    (2),
 
     where
         θ 1  is the diffraction angle of a light beam having a diffraction order providing the maximum diffraction efficiency at the wavelength λ 1 , and   θ 2  is the diffraction angle of a light beam having a diffraction order providing the maximum diffraction efficiency at the wavelength λ 2 .

CROSS-REFERENCE TO RELATED APPLICATION

This application is a U.S. continuation application filed under 35 USC111(a) claiming benefit under 35 USC 120 and 365(c) of PCT applicationJP2010/005696, filed Sep. 17, 2010, which claims priority to JapanesePatent Application No. 2009-216227, filed on Sep. 17, 2009. Theforegoing applications are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens element for use in anoptical pickup device.

2. Description of the Background Art

As media that record a large amount of information with a high density,there are optical information storage media such as optical discs.Optical discs record information as pit- shaped patterns thereon, andare widely used for the purposes of recording digital audio files, videofiles, document files, and data files. Examples of functions requiredfor performing recording, reproducing, and erasing of information on anoptical disc with high reliability by using a light beam converged on amicro spot are a converging function to form a diffraction- limitedmicro spot, focus control (focus servo) of an optical system, trackingcontrol, and pit signal (information signal) detection.

In recent years, due to advancement of optical system design technologyand shortening of the wavelengths of semiconductor lasers which arelight sources, development has progressed concerning optical discs thathave a higher-density storage capacity further than ever. One approachto density increase is to increase the optical disc-side numericalaperture (NA) of a light-converging optical system which converges alight beam to form a micro spot on the optical disc. However, when theNA of the light-converging optical system is increased, an amount of agenerated aberration increases with respect to a certain amount of tiltof the optical axis. In order to prevent this problem, it is necessaryto decrease the thickness of a layer (hereinafter, referred to as “basematerial thickness”) provided on a recording surface of the opticaldisc. In the present specification, the “base material thickness” meansa thickness from a light beam incident surface to an informationrecording surface of an optical disc.

For compact discs (CD) which are first generation optical discs,infrared light (a wavelength λ₃: 780 to 820 mu) and an objective lenshaving an NA of 0.45 are used. The base material thickness of CD is 1.2mm. For DVD which is second generation, red light (a wavelength λ₂: 630to 680 nm) and an objective lens having an NA of 0.6 are used. The basematerial thickness of DVD is 0.6 mm. For third generation optical discs,blue light (a wavelength λ₁: 390 to 415 nm) and an objective lens havingan NA of 0.85 are used. The base material thickness of third generationoptical discs is 0.1 mm. As described above, as the recording densityincreases, the base material thickness of the optical disc decreases.

In view of economical efficiency and space occupied by an apparatus, anoptical information recording/reproducing apparatus is desired which canperform recording and reproducing on optical discs having different basematerial thicknesses and recording densities. For this, alight-converging optical system which can converge a light beam to adiffraction limit on a recording surface of each of optical discs havingdifferent base material thicknesses, and an optical pickup deviceincluding this light-converging optical system, are necessary. Inaddition, when recording and reproducing are performed on an opticaldisc having a thick base material, it is necessary to converge a lightbeam on a recording surface located deeper than a beam incident surfaceof the optical disc, and thus the focal length has to be increased.

Prior art documents disclose configurations intended for compatiblereproducing and compatible recording on an optical disc having a basematerial thickness of 0.6 mm and designed for the wavelength λ₂ (redlight) and on an optical disc having a base material thickness of 0.1 mmand designed for the wavelength λ₁ (blue light).

A first prior art example is a configuration in which awavelength-selective phase plate is combined with an objective lens.This is disclosed in Japanese Laid-Open Patent Publication No. 10-334504and the Proceedings of ISOM2001 (Session We-C-05), P 30.

As a second prior art example, a configuration in which a refractiontype objective lens and a diffraction element are combined is disclosed.In Japanese Laid-Open Patent Publication No. 2004-071134, in an opticalhead device which performs recording or reproducing on a high-densityoptical disc by using an objective lens having a high NA, asawtooth-like diffraction element is used in order to be able to alsoperform recording or reproducing on conventional optical discs such asDVD. The sawtooth height is set such that when blue light is used, thelength of the optical path becomes 2λ, and 2nd order diffracted light isused. The sawtooth-like diffraction element emits 1st order diffractedlight when red light is incident thereon. The braze direction is as in aconvex lens type, and chromatic aberration compensation of therefractive lens is performed. The diffraction order when red light isused is. lower than the diffraction order when blue light is used. Thus,the sawtooth-like diffraction element serves as a concave lens for redlight, thereby providing an effect that the working distance can beincreased.

Further, Japanese Laid-Open Patent Publication No. 2004-071134 disclosesa stair-like step structure which provides an optical path differencelonger than one wavelength to blue light and which provides an opticalpath difference shorter than one wavelength to red light. The stair-likestep structure also exerts a convex lens effect on blue light and exertsa concave lens effect on red light. Thus, when blue light is used, achromatic aberration compensation effect of the refractive lens isexerted, and when red light is used, an effect that the working distance(the interval between the objective lens surface and the surface of anoptical disc) can be increased is obtained due to the concave lenseffect.

As a third conventional art example, a configuration in which a relaylens is inserted between an infrared light source and an objective lens,thereby also realizing compatibility with a first generation opticaldisc having a base material thickness of 1.2 mm, is disclosed inJapanese Laid-Open Patent Publication No. 2004-281034.

Japanese Laid-Open Patent Publication Nos. 10-334504 and 2004-071134merely disclose the method for compatibility with the above secondgeneration optical discs and the above third generation optical discs.In addition, Japanese Laid-Open Patent Publication No. 2004-281034discloses the method for compatibility with the above first generationoptical discs, but requires a relay lens.

Further, it is desired that an element that realizes compatibility isintegrally formed on the objective lens surface, in view of costreduction by decrease in number of parts. However, in the prior artdescribed above, only the exemplary configuration, in which the phaseplate or the diffraction element is provided independently of therefraction type objective lens, is disclosed, and there is nodescription about integrally forming an element, which realizescompatibility, on the objective lens surface.

Moreover, in order to produce objective lenses at low cost and in largequantities, the material of the objective lenses is preferably resinrather than glass. In general, the material cost of resin is low, and itis also possible to mold resin at a lower temperature than to moldglass. Thus, the mold can be used long and the molding time can beshortened. Therefore, by molding resin to produce objective lenses, themanufacturing cost can be reduced. However, the refractive index of ahigh-NA objective lens made of resin changes due to temperature change.The refractive index change causes the refractive power of the lenssurface to shift from a designed value, whereby a spherical aberrationoccurs. A lower-order aberration greatly deteriorates the quality of aninformation reproduction signal, and thus a 3rd order sphericalaberration is problematic.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an objective lenselement having excellent compatibility with optical discs havingdifferent base material thicknesses.

The present invention is directed to an objective lens element which hasoptically functional surfaces on an incident side and an exit side,which converges a first incident light beam of a wavelength λ₁ through abase plate having a thickness t₁ to form a spot, and which converges asecond incident light beam of a wavelength λ₂ longer than the wavelengthλ₁ through a base plate having a thickness t₂ larger than the thicknesst₁ to form a spot.

In the objective lens element of the present invention, at least eitherone of the optically functional surfaces on the incident side and theexit side includes a diffraction portion which satisfies at least eitherone of the following formulas (1) and (2):

θ₁×θ₂<0 (“×” represents multiplication)   (1), and

(sin θ₂)/λ₂=−(sin θ₁) /λ₁   (2),

where

θ₁ is the diffraction angle of a light beam having a diffraction orderproviding the maximum diffraction efficiency at the wavelength λ₁(θ₂≠0), and

θ₂ (≠0) is the diffraction angle of a light beam having a diffractionorder providing the maximum diffraction efficiency at the wavelength λ₂(θ₂≠0).

Alternatively, in the objective lens element of the present invention,at least either one of the optically functional surfaces on the incidentside and the exit side includes a diffraction portion which satisfiesthe following formula (3):

Φ₂<Φ₀₂<Φ₀₁<Φ₁   (3),

where

Φ₁ is the power of a surface acting on a light beam having a diffractionorder providing the maximum diffraction efficiency at the wavelength λ₁(Φ_(b ≠0)),

Φ₂ is the power of the surface acting on a light beam having adiffraction order providing the maximum diffraction efficiency at thewavelength λ₂ (Φ₂≠0),

Φ₀₁ is the power of a base refractive surface obtained by removing powerby diffraction from Φ₁, and

Φ₀₂ is the power of the base refractive surface obtained by removingpower by diffraction from Φ₂.

According to the present invention, an objective lens element havingexcellent compatibility with optical discs having different basematerial thicknesses can be realized.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram illustrating a diffraction structure formed on asurface of an objective lens element according to Embodiment 1;

FIG. 1B is a diagram illustrating a diffraction structure formed on asurface of an objective lens element according to Embodiment 1;

FIG. 1C is a diagram illustrating a diffraction structure formed on asurface of an objective lens element according to Embodiment 1;

FIG. 2 is a diagram showing the objective lens element according toEmbodiment 1;

FIG. 3 is a diagram showing an example of a step structure provided onan outer part;

FIG. 4A is a diagram showing an example of a sawtooth-like diffractionstructure provided on the outer part;

FIG. 4B is a diagram showing an example of a sawtooth-like diffractionstructure provided on the outer part;

FIG. 4C is a diagram showing an example of a sawtooth-like diffractionstructure provided on the outer part;

FIG. 5 is a configuration diagram of an optical pickup device accordingto Embodiment 2;

FIG. 6 is a diagram showing in detail an objective lens used in theoptical pickup device according to Embodiment 2;

FIG. 7 is optical path diagrams of an objective lens element accordingto Numerical Example 1;

FIG. 8 is graphs showing spherical aberrations of the objective lenselement according to Numerical Example 1;

FIG. 9 is graphs showing sine conditions of the objective lens elementaccording to Numerical Example 1;

FIG. 10 is optical path diagrams of an objective lens element accordingto Numerical Example 2;

FIG. 11 is graphs showing spherical aberrations of the objective lenselement according to Numerical Example 2; and

FIG. 12 is graphs showing sine conditions of the objective lens elementaccording to Numerical Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) Compatibilitywith BD and DVD

FIG. 1A to 1C is a diagram illustrating a diffraction structure formedon a surface of an objective lens element according to Embodiment 1.Specifically, FIG. 1A is a diagram illustrating the physical shape ofthe diffraction structure, and FIG. 1B shows an amount of phase changeprovided by the diffraction structure shown in FIG. 1A to light of awavelength λ₂ (red light). FIG. 1C shows an amount of phase changeprovided by the cross-sectional shape shown in FIG. 1A to light of awavelength λ₁ (blue light). The steps of FIG. 1A are formed on a basesurface which is a refractive surface (aspherical shape) in the surfaceof the objective lens element.

In FIG. 1A, the vertical direction indicates the thickness (height) ofthe lens material in the optical axis direction. Hereinafter, the shapeshown in FIG. 1A is referred to as “binary diffraction structure”. Forexample, when a polyolefin resin is used as the material of theobjective lens element, a material having a refractive index n_(b) ofabout 1.522 with respect to the blue light of the wavelength λ₁ can beused. A step d₁ (hereinafter, also referred to as “unit step”) providesan optical path difference of about 1.25 wavelength, or a phasedifference of about (2π+π/2), to the light of the wavelength λ₁. As anexample, where λ₁ is 405 nm and n_(b) is 1.522, the height of the unitstep d₁ is λ₁/(n_(b)−1)×1.25=0.97 μm.

It should be noted that the “optical path difference” means thedifference between the length of an optical path in the case where thesteps are present (the medium of the step portion is the lens material)and the length of an optical path in the case where no steps are present(the medium of the step portion is air).

FIG. 1A shows that the optical path difference caused by the unit stepd₁ is 1.25 times that of the blue wavelength λ₁. The optical pathdifference caused by the unit step d₁ is obtained by stepheight/(n_(b)−1), and thus 1.25 is a value obtained by dividing stepheight/(n_(b)−1) by λ₁.

Where the unit step is d₁, the height (level) from the base surface isan integral multiple of d₁. Thus, an amount of phase change provided bythe step shape to the light of the wavelength λ₁ is an integral multipleof (2π+π2). This substantially means that the amount of phase changechanges by π/2 per step each time the height from the base surfaceincreases by one unit step as shown in FIG. 1C.

A lens material that is a polyolefin resin has a refractive index n_(r)of about 1.505 with respect to the light of the wavelength λ₂. Theoptical path difference provided by the step d, to the light of thewavelength λ₂ can be represented by d₁×(n_(r) −1). Where λ₁ is 405 nm,λ₂ is 650 nm, n_(b) is 1.522, and n_(r)is 1.505, the optical pathdifference corresponds to about 0.75 wavelength, and this means that theamount of phase change changes by −π/2 each time the height from thebase surface increases by one step (d₁).

When the height of each step structure from the base surface is anintegral multiple of d₁ and a stair-like cross-sectional shape isprovided as shown in FIG. 1A, the amount of phase change provided to thelight of the wavelength λ₁ changes by π/2 per step as shown in FIG. 1C.In other words, the optical path difference changes in steps of +¼wavelength.

Meanwhile, as shown in FIG. 1B, the amount of phase change provided tothe light of the wavelength λ₂ changes by −π/2 per step. In other words,the optical path difference changes in steps of −¼ wavelength. Theamount of phase change is positive when the light of the wavelength λ₁is used, while the amount of phase change is negative when the light ofthe wavelength λ₂ is used. This means that the light of the wavelengthλ₁ and the light of the wavelength λ₂ are subjected to the oppositeactions from the steps.

For example, when the intervals at which the phase steps are formed areappropriately set such that a convex lens effect is exerted on the lightof the wavelength λ₁, a concave lens effect is exerted on the light ofthe wavelength λ₂. Then, the focal point of the light of the wavelengthλ₁ gets close to the objective lens element, while the focal point ofthe light of the wavelength λ₂ moves away from the objective lenselement. Thus, an effect is obtained that the light of the wavelength λ₂can be converged on an information recording surface through a thickerbase material.

FIG. 2 is a diagram illustrating the objective lens element according toEmbodiment 1.

On an optically functional surface of the objective lens element 141 onan incident side, an inner part 131B including a rotational symmetryaxis and a ring-shaped outer part 131F surrounding the inner part 131Bare provided. When the light of the wavelength λ₁ is used, the objectivelens element 141 converges first incident light 61 incident on both theinner part 131B and the outer part 131F, and forms a spot on aninformation recording surface 91 of an optical disc 9 through a basematerial having a thickness t₁. In addition, when the light of thewavelength λ₂ is used, the objective lens element 141 converges secondincident light 62 incident on the inner part 131B, and forms a spot onan information recording surface 101 of an optical disc 10 through abase material having a thickness t₂. The outer part 131F is a regionwhich substantially does not contribute to spot formation when the lightof the wavelength λ₂ is used. Where the numerical aperture defined whenthe first incident light 61 is converged on the information recordingsurface 91 of the optical disc 9 is NA₁ and the numerical aperturedefined when the second incident light 62 is converged on theinformation recording surface 101 of the optical disc 10 is NA₂, NA₁ isequal to or higher than 0.85 and NA₂ is equal to or higher than 0.6.

In other words, the inner part 131B is a region shared by the light ofthe wavelength λ₁ and the light of the wavelength λ₂, and the outer part131F is a region dedicated for the light of the wavelength λ₁. On theinner part 131B, the stair-like step structure shown in FIG. 1A isformed.

FIG. 3 is a diagram showing an example of a step structure provided onthe outer part, and FIG. 4 is a diagram showing an example of asawtooth-like diffraction structure provided on the outer part.

A sawtooth-like diffraction structure may be formed on the outer part131F, and designing may be performed such that the light of thewavelength λ₁ incident on the outer part 131F is converged on theinformation recording surface 101 through the base material having thethickness t₁, and the light of the wavelength λ₂ incident on the outerpart 131F causes a great aberration and is substantially not convergedon the information recording surface 91.

Alternatively, a stair-like binary diffraction structure which is thesame as that on the inner part 131B may be provided on the outer part131F.

Still alternatively, no steps may be formed on the outer part 131F, anddesigning may be performed such that the light of the wavelength λ₁incident on the outer part 131F is converged on the informationrecording surface 101 through the base material having the thickness t₁,and the light of the wavelength λ₂ incident on the outer part 131Fcauses a great aberration and is substantially not converged on theinformation recording surface 91.

Still alternatively, steps each of which causes an optical pathdifference of 1 wavelength of the wavelength λ₁ may be formed on theouter part 131F as shown in FIG. 3. Each of the steps substantially doesnot provide a phase difference to the light of the wavelength λ₁, andthus the light of the wavelength λ₁ incident on the outer part 131F isconverged on the information recording surface 101 through the basematerial having the thickness t₁. Meanwhile, each of the steps providesa phase difference of about 0.6 wavelength to the light of thewavelength λ₂, and thus the light of the wavelength λ₂ incident on theouter part 131F causes a great aberration and is substantially notconverged on the information recording surface 91.

Still alternatively, a sawtooth-like diffraction grating may be formedon the outer part 131F, and designing may be performed such that thelight of the wavelength λ₁ incident on the outer part 131F is convergedon the information recording surface 101 through the base materialhaving the thickness t₁ and the light of the wavelength λ₂ incident onthe outer part 131F is not converged on the information recordingsurface 91. In this case, the sawtooth height suffices to be set to sucha height that an optical path difference of 1 wavelength is caused tothe light of the wavelength λ₁ as shown in FIG. 4A. Since the opticalpath difference provided by the sawtooth to the light of the wavelengthλ₁ is 1 time that of the wavelength λ₁ as shown in FIG. 4B, thediffraction efficiency of the 1st order diffracted light increases, andthe light of the wavelength λ₁ incident on the outer part 131F isconverged on the information recording surface 101 through the basematerial having the thickness t₁. Meanwhile, the optical path differenceprovided by the sawtooth to the light of the wavelength λ₂ is only about0.6 times that of wavelength, and thus the light of the wavelength λ₂ isdiffracted into 1st order diffracted light and zero order diffractedlight and is not converged on a point. In this manner, the sawtooth-likediffraction grating provided on the outer part 131F exerts an aperturelimiting function. The diffraction order providing the maximumdiffraction efficiency of the light of the wavelength λ₁ is notparticularly limited to a specific order.

The material of the objective lens element is not particularly limitedto a specific one. When resin is used, it is possible to performdesigning such that aberration deterioration caused by temperaturechange is suppressed.

Here, conditions which are to be satisfied by the objective lens elementaccording to the present embodiment will be described.

The objective lens element according to the present embodiment satisfiesat least either one of the following condition (1) or (2).

θ₁×θ₂<0(“×” represents multiplication)   (1)

(sin θ₂)/λ₂=−(sin θ₁)/λ₁   (2)

Here,

θ₁ is the diffraction angle of a light beam having a diffraction orderproviding the maximum diffraction efficiency at the wavelength λ₁, and

θ₂ is the diffraction angle of a light beam having a diffraction orderproviding the maximum diffraction efficiency at the wavelength λ₂.

The condition (1) defines that the diffraction direction of the light ofthe wavelength λ₁ is opposite to the diffraction direction of the lightof the wavelength λ₂. As described above, the phase modulation shown inFIG. 1C is performed on the light of the wavelength λ₁. When one unitstructure is taken as being composed of consecutive four steps in FIG. 1A, the step structure of FIG. 1 A can be approximately considered as aperiodic sawtooth-like diffraction structure. A light beam incident on adiffractive surface from the upward direction of the sheet is diffractedleftward at a predetermined diffraction angle θ₁. When the light of thewavelength λ₂ is incident on the same periodic structure, the phasemodulation shown in FIG. 1B is performed. Thus, similarly to the case ofthe wavelength λ₁, when the step structure of FIG. 1A is taken as aperiodic sawtooth-like diffraction structure, a light beam incident onthe diffractive surface from the upward direction of the sheet isdiffracted rightward at a predetermined diffraction angle θ₂.

The condition (2) defines the relationship between wavelength anddiffraction angle. Where the diffraction angle of the light of thewavelength λ₁ is θ₁, the wavelength is λ₁, the diffraction order is m₁order, and the pitch of the unit structure is P, it is satisfied thatsin θ₁=(λ₁/P)×m₁. In addition, where the diffraction angle of the lightof the wavelength λ₂ is θ₂, the wavelength is λ₂, the diffraction orderis m₂ order, and the pitch of the unit structure is P, it is satisfiedthat sin θ₂=(λ₂/P) ×m₂. In the present embodiment, the depth of eachstep is adjusted such that the diffraction order providing the maximumdiffraction efficiency of the light of the wavelength λ₁ is +1st (m₁=1)and the diffraction order providing the maximum diffraction efficiencyof the light of the wavelength λ₂ is −1st (m₂=−1). Here, minus of adiffraction order indicates that the direction of the diffraction orderis opposite to that of the diffraction order +1 of the light of thewavelength λ₁. The condition (2) is derived from these two equations.

The objective lens element according to the present embodiment satisfiesthe following condition (3).

Φ₂<Φ₀₂<Φ₀₁<Φ₁   (3)

Here,

Φ₁ is the power of a surface acting on a light beam having a diffractionorder providing the maximum diffraction efficiency at the wavelength λ₁(Φ₁≠0),

Φ₂ is the power of the surface acting on a light beam having adiffraction order providing the maximum diffraction efficiency at thewavelength λ₂ (Φ₂≠0),

Φ₀₁ is the power of a base refractive surface obtained by removing powerby diffraction from Φ₁, and

Φ₀₂ is the power of the base refractive surface obtained by removingpower by diffraction from Φ₂.

The condition (3) defines the relationship between power by diffraction(including both diffraction and refraction) of the objective lenselement and power by refraction. The power of a lens surface is theinverse of the focal length of the surface. Thus, the condition (3) isequal to the following condition (3)′. The reason why these conditions(3) and (3)′ are satisfied is that designing is performed such that thebinary diffraction structure exerts a convex lens effect on the light ofthe wavelength λ₁ and exerts a concave lens effect on the light of thewavelength λ₂ as described above.

f₁<f₀₁<f₀₂<f₂   (3)′

Here,

f₁ is the focal length for light having a diffraction order providingthe maximum diffraction efficiency at the wavelength λ₁,

f₂ is the focal length for light having a diffraction order providingthe maximum diffraction efficiency at the wavelength λ₂,

f₀₁ is the focal length of the base refractive surface obtained byremoving an influence of diffraction from f₁, and

f₀₂ is the focal length of the base refractive surface obtained byremoving an influence of diffraction from f₂.

In the case of a conventional sawtooth-like diffraction shape, even whenlight having different wavelengths is incident thereon, the signs ofdiffraction angles are not opposite to each other. In other words, whenlight having different wavelengths is incident on a sawtooth-like reliefshape, light of a relatively long wavelength is diffracted at a highdiffraction angle but light of a relatively short wavelength isdiffracted at a diffraction angle lower than this angle. The diffractionangles are different but the directions (signs) of the diffraction arethe same.

In contrast, the diffraction structure according to the presentembodiment can reverse the signs of the diffraction angles for lighthaving different wavelengths. For example, the diffraction structure isuseful for increasing the focal length to ensure a sufficient workingdistance of the objective lens element for DVD.

In general, the level of difficulty in manufacturing an objective lenselement for BD increases as the working distance (WD) increases. Inaddition, in a BD/DVD compatible objective lens element, the basematerial is thicker in DVD than in BD, and thus the focal length isincreased in order to ensure a sufficient working distance in using DVD.In this case, since the working distance in using BD is increased, thelevel of difficulty in manufacturing a lens increases. In theconventional art, a compatible method in which a diffraction structureis provided to create a difference in focal length on the basis of aused wavelength difference has been used. However, in this method, sincea difference in diffraction angle for each used wavelength is utilized,the pitch of diffraction zones has to be very small in order to obtain asufficient working distance for DVD, leading to decrease in diffractionefficiency. On the other hand, due to diffraction power being increased,a chromatic aberration in using BD increases.

In contrast, when the diffraction structure according to the presentinvention is used, a sufficient working distance for DVD can be ensuredwith diffraction zones having a relatively large diffraction pitch. Thediffraction structure according to the present invention can providenegative diffraction power to the light of the wavelength λ₂ for DVD,can provide positive diffraction power to the light of the wavelength λ₁for BD, and can compensate a chromatic aberration at the same time.

(Embodiment 2) Compatibility with BD, DVD, and CD

FIG. 5 is a configuration diagram of an optical pickup device accordingto Embodiment 2. The optical pickup device of FIG. 5 includes a laserbeam source 1 which emits blue light of the wavelength λ₁ (390 nm to 415nm: normally about 408 nm), a laser beam source 20 which selectivelyemits red light of the wavelength λ₂ (630 nm to 680 nm: normally 660 nmis often used) and infrared light of a wavelength λ₃ (770 nm to 810 nm:normally 780 nm), a collimating lens 8, an upward reflection mirror 12which bends an optical axis, and an objective lens element 143.

The optical disc 9 is a third generation optical disc which has a basematerial thickness t, of about 0.1 mm and on which recording orreproducing is performed with a light beam of the wavelength λ₁. Theoptical disc 10 is a second generation optical disc, such as DVD, whichhas a base material thickness t₂ of about 0.6 mm and on which recordingor reproducing is performed with a light beam of the wavelength λ₂. Anoptical disc 11 is a first generation optical disc, such as CD, whichhas a base material thickness t₃ of about 1.2 mm and on which recordingor reproducing is performed with a light beam of the wavelength λ₃. InFIG. 5, portions of the optical discs 9 and 10, namely, only the basematerials from light incident surfaces to recording surfaces, are shown.In reality, each of the optical discs 9 and 10 is attached to aprotective plate in order to reinforce mechanical strength and to havean overall thickness of 1.2 mm which is the same as the thickness of CD.The optical disc 10 is attached to a protective plate having a thicknessof 0.6 mm, and the optical disc 9 is attached to a protective platehaving a thickness of 1.1 mm. A thin protective plate is provided on theoptical disc 11 as well. In FIG. 5, illustration of the protectiveplates is omitted for simplification.

In FIG. 5, the configuration employing the two-wavelength laser beamsource 20 which emits the light of the wavelengths λ₂ and λ₃ is shown.However, a configuration in which different light sources are preparedfor these wavelengths, respectively, and optical paths are combined byusing a dichroic mirror, is also possible.

The laser beam sources 1 and 20 are preferably semiconductor lasersources. By using semiconductor laser sources, the optical pickup deviceand an optical information apparatus employing this optical pickupdevice can be decreased in size, weight, and power consumption.

When recording or reproducing is performed on the highest-recordingdensity optical disc 9, a blue light beam 61 of the wavelength λ₁emitted from the laser beam source 1 is reflected by a beam splitter 4,is converted by the collimating lens 8 into substantially parallellight, and is further converted by a quarter wavelength plate 5 intocircular polarized light. The quarter wavelength plate 5 is designed toserve as a quarter wavelength plate for the light of both wavelengths λ₁and λ₂. The light emitted from the quarter wavelength plate 5 is furtherbent by the upward reflection mirror 12, and is converged by theobjective lens element 143 on the information recording surface throughthe base material of the optical disc 9 which has a thickness of about0.1 mm. Here, for convenience of the drawing, the upward reflectionmirror 12 is shown to bend the light beam in the upward direction of thedrawing. However, in reality, the upward reflection mirror 12 isdisposed so as to bend the light beam in a direction orthogonal to thesheet.

The blue light beam 61reflected by the information recording surfacetravels along the optical path in the reverse direction and is convertedby the quarter wavelength plate 5 into linearly polarized light having apolarization plane orthogonal to the polarization plane of the initiallinearly polarized light. The light emitted from the quarter wavelengthplate 5 almost totally passes through the beam splitter 4, is totallyreflected by a beam splitter 16, is diffracted by a detectiondiffraction element 31 is further converged by a detection lens 32, andis incident on a photodetector 33. Output of the photodetector 33 issubjected to arithmetic processing to obtain a servo signal and aninformation signal which are used for focus control and trackingcontrol. The beam splitter 4 includes a polarization splitting filmwhich, with regard to a light beam of the wavelength λ₁, totallyreflects linearly polarized light having a certain direction and totallypasses linearly polarized light having a direction orthogonal to thecertain direction, as described above. In addition, as described later,the polarization splitting film totally passes a red light beam 62 andinfrared light which are emitted from the laser beam source 20. Asdescribed above, the beam splitter 4 is an optical path branchingelement which has polarization properties as well as wavelengthselectivity. It is also possible to eliminate the polarizationdependency of the beam splitter 4 and to omit the quarter wavelengthplate 5.

Next, when recording or reproducing is performed on the optical disc 10,a light beam of the wavelength λ₂ which is substantially linearlypolarized light emitted from the laser beam source 20 passes through thebeam splitter 16 and the beam splitter 4 and is converted by thecollimating lens 8 into substantially parallel light. The light beamemitted from the collimating lens 8 is bent by the upward reflectionmirror 12 and is converged by the objective lens element 143 on theinformation recording surface through the base material of the opticaldisc 10 which has a thickness of about 0.6 mm.

The light beam reflected by the information recording surface travelsalong the optical path in the reverse direction, almost totally passesthrough the beam splitter 4, is totally reflected by the beam splitter16, is diffracted by the detection diffraction element 31, is convergedby the detection lens 32, and is incident on the photodetector 33.Output of the photodetector 33 is subjected to arithmetic processing toobtain a servo signal and an information signal which are used for focuscontrol and tracking control. In order to obtain servo signals for theoptical disc 9 and 10 from the common photodetector 33 as describedabove, the light-emitting point of the laser beam source 1 and the redlight-emitting point of the laser beam source 20 are located in aconstruct-image relation with respect to a common position on theobjective lens 143. When such a configuration is provided, the number ofdetectors and the number of wires can be reduced.

The beam splitter 16 includes a polarization splitting film which, withregard to light of the wavelength λ₂, totally passes linearly polarizedlight having a certain direction and totally reflects linearly polarizedlight having a direction orthogonal to the certain direction. The beamsplitter 16 totally passes the blue light beam 61 of the wavelength λ₁.As described above, the beam splitter 16 is also an optical pathbranching element which has polarization properties as well aswavelength selectivity. It is also possible to eliminate thepolarization dependency of the beam splitter 16 and to omit the quarterwavelength plate 5. The optical path when the laser beam source 20 iscaused to emit infrared light to perform recording or reproducing on theoptical disc 11 is the same as that when the light source 20 is causedto emit red light of the wavelength λ₂ to perform recording orreproducing on the optical disc 10.

As shown in FIG. 5, a three-beam grating (diffraction element) 3 may bedisposed between the laser beam source 1 and the beam splitter 4. Inthis case, it is possible to detect a tracking error signal of theoptical disc 9 by the well-known differential push-pull (DPP) method. Inaddition, a relay lens 2 can be disposed between the laser beam source 1and the beam splitter 4 to set the numerical aperture on the collimatinglens 8 side of the light beam 61 to an appropriate value.

Moreover, a three-beam grating (diffraction element) 22 may be disposedbetween the laser beam source 20 and the beam splitter 16. In this case,it is possible to detect a tracking error signal of the optical disc 10by the well-known differential push-pull (DPP) method. In addition, itis effective to change the parallelism of a light beam by moving thecollimating lens 8 along the optical axis direction (the right-leftdirection in FIG. 5). A spherical aberration occurs due to a thicknesserror of the base material. When the optical disc 9 has multilayerinformation recording surfaces, a spherical aberration occurs due to adifference of the base material thickness for each information recordingsurface. These spherical aberrations can be compensated by moving thecollimating lens 8 along the optical axis direction.

Spherical aberration compensation performed by moving the collimatinglens 8 is possible with about several hundreds mλ when the NA is 0.85,and a spherical aberration corresponding to the fluctuation range ofbase material thickness of ±30 μm can also be compensated. In addition,when recording or reproducing is performed on the optical disc 11 byusing the infrared light beam, the collimating lens 8 can be movedtoward the left side of FIG. 5, namely, toward the laser beam source 20to convert a light beam travelling toward the objective lens element143, into diverging light. Thus, a convergence spot formed on aninformation recording surface of the optical disc 11 can be movedfurther away from the objective lens element 143, a part of anaberration caused by the base material thickness can be compensated, anaberration compensation amount required for the optical element 131 canbe reduced to reduce the number of steps, and hence production of anoptical element can be made easy.

Moreover, the beam splitter 4 may pass a portion (e.g., about 10%) ofthe linearly polarized light of the wavelength λ₁ emitted from the laserbeam source 1. The passed light beam is guided by a converging lens 6 toa photodetector 7. A signal obtained from the photodetector 7 is used tomonitor change in amount of the light emitted by the laser beam source1, and the change in amount of the light is fed back, whereby controlcan be performed such that the amount of the light emitted by the laserbeam source 1 is kept constant.

Moreover, the beam splitter 4 may reflect a portion (e.g., about 10%) ofthe linearly polarized light emitted from the laser beam source 20. Thereflected light beam is guided by the converging lens 6 to thephotodetector 7. A signal obtained from the photodetector 7 is used tomonitor change in amount of the light emitted by the laser beam source20, and the change in amount of the light is fed back, whereby controlcan be performed such that the amount of the light emitted by the laserbeam source 20 is kept constant.

FIG. 6 is a diagram showing in details an object lens element used inthe optical pickup device according to Embodiment 2. The objective lenselement 143 has a first surface on an incident side and a second surfaceon an exit side. The first surface is divided into three concentricregions, and different diffraction structures are formed on theseregions, respectively. The second surface is divided into two concentricregions, and different aspheric surfaces are formed on these regions,respectively.

Specifically, the first surface is divided into an inner region 151Bincluding the optical axis, an intermediate region 151M surrounding theinner region 151B, and an outer region 151F surrounding the intermediateregion 151M. A stair-like binary diffraction structure consisting ofeight steps is formed on the inner region 151B. The depth of each stepis set so as to correspond to an optical path length of 1.25 wavelengthsof the wavelength λ₁ of the blue light. In this case, the diffractionorder of a light beam having the highest diffraction efficiency is +2ndorder for the blue light of the wavelength λ₁, −2nd order for the redlight of the wavelength λ₂, and −3rd order for the infrared light of thewavelength λ₃.

A stair-like binary diffraction structure consisting of four steps isformed on the intermediate region 151M. The depth of each step is set soas to correspond to an optical path length of 1.25 wavelengths of thewavelength λ₁ of the blue light. In this case, the diffraction order ofa light beam having the highest diffraction efficiency is +1st order forthe blue light of the wavelength λ₁ and −1st order for the red light ofthe wavelength λ₂.

A conventional sawtooth-shaped diffraction structure is formed on theouter region 151F. The depth of each sawtooth is set so as to correspondto an optical path length of 3 wavelengths of the wavelength λ₁ of theblue light. The depth is not limited to 3 wavelengths and suffices to bean integral multiple of the wavelength λ₁. Alternatively, the outerregion 151F may not be the sawtooth-shaped diffraction structure and maybe an aspheric surface or a binary diffractive surface.

The objective lens element 143 according to the present embodiment isadvantageous in that a sufficient working distance is easily ensured,since the sign of the diffraction angle of the blue light of thewavelength λ₁ is opposite to the signs of the diffraction angles of thered light of the wavelength λ₂ and the infrared light of the wavelengthλ₃.

Here, the objective lens element 143 according to the present embodimentsatisfies the following condition (4).

Φ₃<Φ₂<Φ₀₃<Φ₀₂<Φ₀₁<Φ₁   (4)

Here,

Φ₁ is the power of a surface acting on a light beam having a diffractionorder providing the maximum diffraction efficiency at the wavelength λ₁(Φ₁≠0),

Φ₂ is the power of the surface acting on a light beam having adiffraction order providing the maximum diffraction efficiency at thewavelength λ₂ (Φ₂≠0),

Φ₃ is the power of the surface acting on a light beam having adiffraction order providing the maximum diffraction efficiency at thewavelength λ₃ (Φ₃≠0),

Φ₀₁ is the power of a base refractive surface obtained by removing powerby diffraction from Φ₁,

Φ₀₂ is the power of the base refractive surface obtained by removingpower by diffraction from Φ₂, and

Φ₀₃ is the power of the base refractive surface obtained by removingpower by diffraction from Φ₃.

The condition (4) defines the relationship between power by diffraction(including both diffraction and refraction) of the objective lenselement and power by refraction. The power of a lens surface is theinverse of the focal length of the surface. Thus, the condition (4) isequal to the following condition (4)′. The reason why these conditions(4) and (4)′ are satisfied is that designing is performed such that thebinary diffraction structure exerts a convex lens effect on the light ofthe wavelength λ₁ and exerts a concave lens effect on the light of thewavelengths λ₂ and λ₃.

f₁<f₀₁<f₀₂<f₀₃<f₂<f₃   (4)′

Here,

f₁ is the focal length for light having a diffraction order providingthe maximum diffraction efficiency at the wavelength λ₁,

f₂ is the focal length for light having a diffraction order providingthe maximum diffraction efficiency at the wavelength λ₂,

f₃ is the focal length for light having a diffraction order providingthe maximum diffraction efficiency at the wavelength λ₃,

f₀₁ is the focal length of the base refractive surface obtained byremoving an influence of diffraction from f₁,

f₀₂ is the focal length of the base refractive surface obtained byremoving an influence of diffraction from f₂, and

f₀₃ is the focal length of the base refractive surface obtained byremoving an influence of diffraction from f₃.

EXAMPLES

Hereinafter, numerical Examples of the present invention will bespecifically described with construction data, aberration diagrams, andthe like. In each Numerical Example, a surface to which an asphericcoefficient is provided indicates a refractive optical surface having anaspherical shape or a surface (e.g., a diffractive surface) having arefraction function equal to that of an aspheric surface. The surfaceshape of an aspheric surface is defined by the following formula 1.

$X = {\frac{C_{j}h^{2}}{1 + \sqrt{1 - {\left( {1 + k_{j}} \right)C_{j}^{2}h^{2}}}} + {\sum\; {A_{j,n}h^{n}}}}$

Here,

X is the distance from an on-the-aspheric-surface point at a height hrelative to the optical axis to a tangential plane at the top of theaspheric surface,

h is the height relative to the optical axis,

C_(j) is the radius of curvature at the top of an aspheric surface of alens jth surface (C_(j)=1/R_(j)),

K_(j) is the conic constant of the lens jth surface, and

A_(j,n) is the nth-order aspheric constant of the lens jth surface.

Further, a phase difference caused by a diffraction structure added toan optical surface is provided by the following formula 2.

φ(h)=ΣP _(j,m) h ^(2m)

The meaning of each character in the formula 2 is as follows:

Φ(h) is a phase function,

h is the height relative to the optical axis, and

P_(j,m) is the 2mth-order phase function coefficient of the lens jthsurface.

Numerical Example 1

Numerical Example 1 corresponds to Embodiment 1 shown in FIG. 2. Tables1 to 4 show construction data of an objective lens element according toNumerical Example 1.

TABLE 1 BD DVD Wavelength 0.408 0.658 Effective diameter 2.24 1.74 NA0.86 0.6 Working distance (WD) 0.4 0.3 Disc thickness (DT) 0.1 0.6 Focallength 1.3 1.4 Diffraction order of first 2 −2 region on first surfaceDiffraction order of second 1 — region on first surface Object point(OP) ∞ 100

TABLE 2 Radius of curvature at Surface the top of No. lens surfaceThickness Material Remarks 0 OP 1 0.8623596 1.53761 n1 First region(diffractive surface), second region (diffractive surface) 2−1.412713    WD First region (aspherical surface), second region(aspherical surface) 3 ∞ DT Disc Planar 4 ∞ Planar

TABLE 3 Wavelength 0.408 0.658 n1 1.52183 1.50399 Disc 1.61642 1.57829

TABLE 4 First region, First surface diffractive surface Diffractivesurface Region 0 mm to 0.875 mm Aspherical constants RD 0.8623596 k−0.60941585 A0 0 A2 0 A4 0.030312057 A6 0.007903167 A8 0.033434594 A10−0.040242123 A12 0.03565307 First region, First surface phase functionDiffractive surface P2 −126.34959 P4 9.7443596 P6 −3.0387489 Secondregion, First surface diffractive surface Diffractive surface Region0.875 mm to 1.135 mm Aspherical constants RD 0.88351059  k−0.55670259    A0 6.19E−05 A2 0       A4 0.031839006 A6 0.026003687 A80.014891698 A10 0.005484338 A12 −0.000871703   A14 −0.004931886   A16−0.00766895    Second region, First surface phase function Diffractivesurface P2 −130.16042 P4 8.8588727 P6 3.7653304 First region, Secondsurface diffractive surface Diffractive surface Region 0 mm to 0.53 mmAspherical constants RD −1.4180252 k −23.75474 A0 0 A2 0 A4 0.35949876A6 −0.28463298 A8 −3.2713988 A10 19.065115 A12 −33.47043 Second region,Second surface diffractive surface Diffractive surface Region 0.53 mm to0.88 mm Aspherical constants RD −2.6229919 k −99.799757 A0 −0.014713257A2 0 A4 0.026112683 A6 −0.019911979 A8 −0.059772709 A10 −0.018229013 A120.084205932 A14 0.1033072 A16 −0.13955802

A first surface of the objective lens element according to NumericalExample 1 is divided into an inner region (first region) and an outerregion (second region). A binary diffractive surface is provided on theinner region including the optical axis, and a sawtooth-like diffractivesurface is provided on the outer region. A second surface is dividedinto an inner region (first region) and an outer region (second region),and the inner region and the outer region are formed by differentaspheric surfaces, respectively. A peripheral wall portion having arotary symmetry axis which coincides with the optical axis is formed atthe outer periphery of the objective lens element. The objective lenselement according to Numerical Example 1 is compatible with BD and DVD.For BD, the designed wavelength is 408 nm, the focal length is 1.3 mm,the numerical aperture (NA) is 0.86, and the protective layer thicknessof an information storage medium is 0.1 mm. For DVD, the designedwavelength is 658 nm, the focal length is 1.4 mm, the NA is 0.6, and theprotective layer thickness of an information storage medium is 0.6 mm.

FIG. 7 is optical path diagrams of the objective lens element accordingto Numerical Example 1. FIG. 8 is graphs showing spherical aberrationsof the objective lens element according to Numerical Example 1. FIG. 9is graphs showing sine conditions of the objective lens elementaccording to Numerical Example 1. In FIGS. 8 and 9, the graphs for BDshow data obtained when parallel light is incident on the objective lenselement and a spot is formed through a protective layer having athickness of 0.1 mm. Meanwhile, the graphs for DVD show data obtainedwhen diverging light (a virtual object point distance of 100 mm) isincident on the objective lens element and a spot is formed through aprotective layer having a thickness of 0.6 mm. During recording orreproducing on DVD, the position of the collimating lens is moved alongthe optical axis direction to cause diverging light to be incident onthe objective lens element. The virtual object point distance refers toan object point distance which is determined when it is assumed that thediverging light is emitted from a light source, not from a collimatinglens. As seen from FIGS. 8 and 9, aberrations are favorably compensated.

Table 5 shows values obtained from the optical specifications accordingto Numerical Example 1.

TABLE 5 408 nm 658 nm Diffraction angle deg 0.66 −1.09 Refraction angledeg 23.05 23.06 Power of diffractive surface 1/mm 0.64 0.53 Power ofrefractive surface 1/mm 0.605 0.584 Focal length of lens mm 1.30 1.44Focal length of lens (excluding mm 1.34 1.37 diffraction)

The diffraction angle and the refraction angle in Table 5 are valuesobtained for a light beam incident on the position of a radius of 0.872mm in the inner region of the first surface of the objective lenselement. At a wavelength of 408 nm, 2nd diffraction order light is lighthaving the maximum diffraction efficiency, and at a wavelength of 658nm, −2nd diffraction order light is light having the maximum diffractionefficiency.

Here, the positive sign of a diffraction order is defined to represent adirection of diffraction toward the lens inner side. The “diffractionangle” does not include an angle of refraction of a base asphericsurface and indicates an angle of bending only by diffraction. The “baseaspheric surface” refers to an aspheric shape defined by an asphericcoefficient in the construction data shown in Tables 1 to 4.

The “refraction angle” refers to an angle change caused by therefraction effect of a base aspheric surface shape. The “power of adiffractive surface” refers to the power of a surface on which adiffraction structure is provided, and is indicated by the sum of powerby diffraction and power by refraction. Where the focal length of thesurface is f, the “power” is indicated by 1/f. The “power of arefractive surface” refers to the power obtained by removing power bydiffraction from the power of the diffractive surface. In other words,the “power of a refractive surface” refers to power by refraction by abase aspheric surface, and is the same as the power of the surface whenthe diffraction order is zero order. The “lens focal length” refers tothe focal length determined by the effects of both diffraction andrefraction. The “lens focal length (excluding diffraction)” refers tothe lens focal length determined only by the refraction effect by thebase aspheric surface, and is the same as the lens focal length when thediffraction order is zero order.

Further, in the objective lens element according to Numerical Example 1,a step structure is provided in accordance with the range of phase Φ(r)obtained from the phase function. The relationship between the range ofphase Φ(r) and the height of each step provided on the inner region isas follows.

In regions satisfying 2nπ≦Φ(r)≦(2nπ+π/2 ): −1.875 wavelengths

In regions satisfying 2nπ+π/2≦Φ(r)≦(2nπ+π): −0.625 wavelength

In regions satisfying 2nπ+π≦Φ(r)≦(2nπ+3π/2): +0.625 wavelength

In regions satisfying 2nπ+3π/2≦Φ(r)≦(2nπ+2π): +1.875 wavelengths

In reality, the step height corresponding to 1 wavelength is representedby λ/(nd−1) [λ:

designed wavelength, nd: the material refractive index with respect tothe wavelength]. Steps obtained by multiplying the step height by eachcoefficient are formed on the base aspheric surface shape.

Further, a sawtooth-like diffraction shape is provided on the outerregion (region dedicated for BD). The outer region is used only for BD,the blaze depth is set so as to be three times of the wavelength for BDsuch that the outer region exerts an aperture limiting function when DVDis used, and the diffraction order of the light of the wavelength for BDis +3rd order. However, the diffraction order of the light of thewavelength for BD may be an order other than +3rd order.

Numerical Example 2

Numerical Example 2 corresponds to Embodiment 2 shown in FIG. 6. Tables6 to 10 show construction data of the objective lens element accordingto Numerical Example 2.

TABLE 6 BD DVD CD Wavelength 0.408 0.658 0.785 Effective diameter 2.241.75 1.49 NA 0.86 0.6 0.47 Working distance (WD) 0.4 0.3 0.035 Discthickness (DT) 0.0875 0.6 1.2 Focal length 1.3 1.4 1.5 Diffraction orderof first 2 −2 −3 region on first surface Diffraction order of second 1−1 — region on first surface Diffraction order of third 3 — — region onfirst surface Object point (OP) ∞ 100 31

TABLE 7 Radius of curvature at Surface the top of No. lens surfaceThickness Material Remarks 0 OP 1 0.85748234 1.537237 n1 First region(diffractive surface), second region (diffractive surface), third region(diffractive surface) 2 −1.412713    WD First region (asphericalsurface), second region (aspherical surface) 3 ∞ DT Disc Planar 4 ∞Planar

TABLE 8 Wavelength 0.408 0.658 0.785 n1 1.52183 1.50399 1.50082 Disc1.61642 1.57829 1.57203

TABLE 9 First region, First surface diffractive surface Diffractivesurface Region 0 mm to 0.744 mm Aspherical constants RD 0.85748234 k−0.60256163 A0 0 A2 0 A4 0.033107557 A6 0.011631507 A8 0.013025021 A100.001288994 A12 0.005707887 First region, First surface phase functionDiffractive surface P2 −133.21672 P4 15.209362 P6 −12.5098225 Secondregion, First surface diffractive surface Diffractive surface Region0.744 mm to 0.874 mm Aspherical constants RD 0.85748234 k −0.60256163 A00 A2 0 A4 0.033107557 A6 0.011631507 A8 0.013025021 A10 0.001288994 A120.005707887 Second region, First surface phase function Diffractivesurface P2 −133.21672 P4 15.209362 P6 −12.5098225 Third region, Firstsurface diffractive surface Diffractive surface Region 0.874 mm to 1.118mm Aspherical constants RD 0.88680595 k −0.57080287 A0 0.016720806 A20.00E+00 A4 0.022217526 A6 0.020728562 A8 0.015644616 A10 0.01020251 A120.004883069 A14 −0.001592452 A16 −0.008783607 A18 −0.006987794 Thirdregion, First surface phase function Diffractive surface P2 −180.40761P4 −15.453924 P6 −117.34886

TABLE 10 First region, Second surface diffractive surface Diffractivesurface Region 0 mm to 0.5 mm Aspherical constants RD −1.4289724 k−28.21094 A0 0 A2 0 A4 0.35962053 A6 −0.21076197 A8 −3.4636071 A1014.626518 A12 −20.43886 Second region, Second surface diffractivesurface Diffractive surface Region 0.5 mm to 0.86 mm Asphericalconstants RD −2.262808 k −133.47989 A0 −0.013717523 A2 0 A4 0.035889372A6 −0.03478197 A8 −0.084319926 A10 −0.016143085 A12 0.10646318 A140.087266298 A16 −0.20318975 A18 0.074148738

A first surface of the objective lens element according to NumericalExample 2 is divided into an inner region (first region), anintermediate region (second region), and an outer region (third region).Different binary diffraction structures are provided on the inner regionand the intermediate region, respectively, of the first surface, and asawtooth-like diffraction structure is provided on the outer region. Asecond surface is divided into an inner region (first region) and anouter region (second region). The inner region and the outer region ofthe second surface are formed by different aspheric surfaces,respectively. A peripheral wall portion having a rotary symmetry axiswhich coincides with the optical axis is formed at the outer peripheryof the objective lens element.

The objective lens element according to Numerical Example 2 iscompatible with BD, DVD, and CD. For BD, the designed wavelength is 408nm, the focal length is 1.3 mm, the numerical aperture (NA) is 0.86, andthe protective layer thickness of an information storage medium is0.0875 mm. Here, the reason why the designed protective layer thicknessis set to 0.0875 mm is to be compatible with a multilayer BD and is thatthe designed protective layer thickness is set as a thickness betweenthe thickest protective layer and the thinnest protective layer. ForDVD, the designed wavelength is 658 nm, the focal length is 1.4 mm, theNA is 0.6, and the protective layer thickness of an information storagemedium is 0.6 mm. For CD, the designed wavelength is 785 nm, the focallength is 1.5 mm, the NA is 0.47, and the protective layer thickness ofan information storage medium is 1.2 mm.

FIG. 10 is optical path diagrams of the objective lens element accordingto Numerical Example 2. FIG. 11 is graphs showing spherical aberrationsof the objective lens element according to Numerical Example 2. FIG. 12is graphs showing sine conditions of the objective lens elementaccording to Numerical Example 2. In FIGS. 11 and 12, the graphs for BDshow data obtained when parallel light is incident on the objective lenselement and a spot is formed through a protective layer having athickness of 0.0875 mm. The graphs for DVD show data obtained whendiverging light (a virtual object point distance of 100 mm) is incidenton the objective lens element and a spot is formed through a protectivelayer having a thickness of 0.6 mm. The graphs for CD show data obtainedwhen diverging light (a virtual object point distance of 31 mm) isincident on the objective lens element and a spot is formed through aprotective layer having a thickness of 1.2 mm. The virtual object pointdistance refers to an object point distance which is determined when itis assumed that the diverging light is emitted from a light source, notfrom a collimating lens. As seen from FIGS. 11 and 12, aberrations arefavorably compensated.

Table 11 shows values obtained from the optical specifications accordingto Numerical Example 2.

TABLE 11 408 nm 658 nm 785 nm Diffraction angle deg 0.7 −1.2 −2.1Refraction angle deg 19.3 18.8 18.4 Power of diffractive surface 1/mm0.03 −0.06 −0.10 Power of refractive surface 1/mm 0.61 0.59 0.58 Focallength of lens mm 1.300 1.448 1.519 Focal length of lens mm 1.338 1.3761.383 (excluding diffraction)

The diffraction angle and the refraction angle in Table 11 are valuesobtained for a light beam which is incident on the position of a radiusof 0.764 mm in the inner region of the first surface of the objectivelens element according to Numerical Example 2 and has a diffractionorder providing the highest diffraction efficiency by the binarydiffraction structure. At a wavelength of 408 nm, 2nd diffraction orderlight is light having the maximum diffraction efficiency; at awavelength of 658 nm, -2nd diffraction order light is light having themaximum diffraction efficiency; and at a wavelength of 785 nm, −3rddiffraction order light is light having the maximum diffractionefficiency.

Here, the positive sign of a diffraction order is defined to represent adirection of diffraction toward the lens inner side. The “diffractionangle” does not include an angle of refraction of a base asphericsurface and indicates an angle of bending only by diffraction. The“refraction angle” refers to an angle change caused by the refractioneffect of a base aspheric surface shape. A stair-like binary structureconsisting of eight steps is provided on the inner region (region sharedby three wavelengths) of the first surface.

In the objective lens element according to Numerical Example 2, therelationship between the height of each step provided on the innerregion of the first surface and the range of phase Φ(r) is as follows.

In regions satisfying 4nπ≦Φ(r)≦(4nπ+π/2): −4.375 wavelengths

In regions satisfying 4nπ+π/2≦Φ(r)≦(4nπ+π): −3.125 wavelengths

In regions satisfying 4nπ+π≦Φ(r)≦(4nπ+3π/2): −1.875 wavelengths

In regions satisfying 4nπ+3π/2≦Φ(r)≦(4nπ+2 π): −0.625 wavelength

In regions satisfying 4nπ+2π≦Φ(r)≦(4nπ+5π/2): +0.625 wavelength

In regions satisfying 4nπ+5π/2≦Φ(r)≦(4nπ+3π): +1.875 wavelengths

In regions satisfying 4nπ+3π≦Φ(r)≦(4nπ+7π/2): +3.125 wavelengths

In regions satisfying 4nπ+7π/2≦Φ(r)≦(4nπ+4π): +4.375 wavelengths

In reality, the step height corresponding to 1 wavelength is representedby λ/(nd−1) [λ: designed wavelength, nd: the material refractive indexwith respect to the wavelength]. Steps obtained by multiplying the stepheight by each coefficient are formed on the base aspheric surfaceshape.

Further, a stair-like binary diffraction structure consisting of foursteps is provided on the intermediate region (region shared by twowavelengths) of the first surface. The relationship between the heightof each step provided on the intermediate region in Numerical Example 2and the range of phase Φ(r) is the same as that for the inner region inNumerical Example 1, and thus the description thereof is omitted.

Further, a sawtooth-like diffraction shape is provided on the outerregion (region dedicated for BD) of the first surface. Here, +1st orderis used as the diffraction order for BD, and thus the blaze depth is setto 1 wavelength of the wavelength for BD.

The present invention can be used for an optical pickup device havingcompatibility with a plurality of types of optical discs havingdifferent base material thicknesses, different compatible wavelengths,and different recoding densities, and a system (e.g., a computer, anoptical disc player, an optical disc recorder, a car navigation system,an editing system, a data server, an AV component, a vehicle, etc.)employing this optical pickup device.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It willbe understood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. An objective lens element which has optically functional surfaces onan incident side and an exit side, which converges a first incidentlight beam of a wavelength λ₁ through a base plate having a thickness t₁to form a spot, and which converges a second incident light beam of awavelength λ₂ longer than the wavelength λ₁ through a base plate havinga thickness t₂ larger than the thickness t₁ to form a spot, wherein atleast either one of the optically functional surfaces on the incidentside and the exit side includes a diffraction portion which satisfies atleast either one of the following formulas (1) and (2):θ₁×θ₂<0(“×” represents multiplication)   (1), and(sin θ₂)/λ₂=−(sin θ₁)/λ₁   (2), where θ₁ is the diffraction angle of alight beam having a diffraction order providing the maximum diffractionefficiency at the wavelength λ₁ (θ₁≠0), and θ₂ is the diffraction angleof a light beam having a diffraction order providing the maximumdiffraction efficiency at the wavelength λ₂ (θ₂≠0).
 2. An objective lenselement which has optically functional surfaces on an incident side andan exit side, which converges a first incident light beam of awavelength λ₁ through a base plate having a thickness t₁ to form a spot,and which converges a second incident light beam of a wavelength λ₂longer than the wavelength λ₁ through a base plate having a thickness t₂larger than the thickness t₁ to form a spot, wherein at least either oneof the optically functional surfaces on the incident side and the exitside includes a diffraction portion which satisfies the followingformula (3):Φ₂<Φ₀₂<Φ₀₁<Φ₁   (3), where Φ₁ is the power of a surface acting on alight beam having a diffraction order providing the maximum diffractionefficiency at the wavelength λ₁ (Φ₁≠0), Φ₂ is the power of the surfaceacting on a light beam having a diffraction order providing the maximumdiffraction efficiency at the wavelength λ₂ (Φ₂≠0), Φ₀₁ is the power ofa base refractive surface obtained by removing power by diffraction fromΦ₁, and Φ₀₂ is the power of the base refractive surface obtained byremoving power by diffraction from Φ₂.
 3. The objective lens elementaccording to claim 2, wherein the objective lens element is furthercapable of converging a third incident light beam of a wavelength λ₃through a base plate having a thickness t₃ to form a spot, and thediffraction portion satisfies the following formula (4):Φ₃<Φ₂<Φ₀₃<Φ₀₂<Φ₀₁<Φ₁   (4), where Φ₃ is the power of the surface actingon a light beam having a diffraction order providing the maximumdiffraction efficiency at the wavelength λ₃ (Φ₃≠0), and Φ₀₃ is the powerof the base refractive surface obtained by removing power by diffractionfrom Φ₃.